Formations

The number of linebackers is dependent upon the formation called for in the play; formations can call for as few as none, or as many as seven. Most defensive schemes call for three or four, and they are named for the number of linemen, followed by the number of linebackers. For example, the 4-3 defense has four defensive linemen and three linebackers; conversely, the 3-4 defense has three linemen and four linebackers.

4-3 defense

In the 4-3 defense there are four down linemen and three linebackers. The middle linebacker is designated "Mike" and two outside linebackers are designated "Sam" and "Will" according to how they line up against the offensive formation. If there is a strong call, the linebacker on the strongside is called "Sam", while the linebacker on the weakside is called "Will". The outside linebacker's job is to cover the end to make sure a run doesn't escape, and to also watch the pass and protect from it. The middle linebacker's job is to stop runs between the Tackles and watch the entire field to see the play develop. On pass plays, the linebackers' responsibilities vary based upon whether a man or zone coverage is called. In a zone coverage, the linebackers will generally drop into hook zones across the middle of the field. However, some zones will send the outside linebackers into the flats. In a man-to-man call, the "Sam" will often cover the tight end with help from a safety over the top, while at other times, the "Sam" and "Will" will be responsible for the first man out of the backfield on their side of the center, with the "Mike" covering if a second man exits on that side of the field.

In the "Tampa 2" zone defense the middle linebacker is required to drop quickly into a deep middle zone pass coverage thus requiring a quick player at this position.

3-4 defense

In the 3-4 Defense there are three lineman playing the line of scrimmage with four linebackers backing them up, typically two outside linebackers and two inside linebackers. The weak side inside linebacker is typically called the "Will," while the strong side inside linebacker is called the "Sam".

The idea behind the 3-4 Defense is to disguise where the fourth rusher will come from. Instead of the standard four down-linemen in the 4-3, only 3 players are clearly attacking on nearly every play. A key for running this defense successfully is having a defensive front of three large defensive linemen who command constant double teams. In particular, the Nose Tackle, who plays over the offensive center, must be able to hold ground and to occupy several offensive blockers in order to allow the linebackers to make plays. The focus of the 3-4 defensive line is to occupy offensive linemen thus freeing the linebackers to tackle the running back or to rush the passer or otherwise drop into pass coverage.

Generally, both outside linebackers can rush the passer and play the run. Outside linebackers in the 3-4 defense tend to be larger in comparison to linebackers in a base 4-3 defense. They are also often players who would play DE in a 4-3 defense as situational pass rushing specialists but who otherwise may not fit the expected role of a DE being somewhat smaller in size. Outside linebackers should be able to drop into pass coverage, rush the passer or read and react. When it comes to the inside linebackers, one is generally a run stuffing player who is better able to handle offensive linemen and stop running backs when the offense features a running play, while the other is often a smaller, faster player who excels in pass coverage. However, the smaller or cover LB should also be able to scrape and plug running lanes decently.

The design concept of the 3-4 defense is to confuse the offensive line in their blocking assignments, particularly in pass blocking, and to create a more complex read for the quarterback. Many 3-4 defenses have the ability to quickly hybrid into a 4-3 on the field. The 3-4 typically has larger, slower players in the front seven than in the respective positions in the 4-3, leaving more pressure on the defensive backs.

46 defense

In the 46 defense, there are four linemen, three linebackers, and a safety who is moved up behind the line of scrimmage. Thus it appears as if there are 4 linebackers, but it is really 3 linebackers with one safety playing up with the other linebackers.
Three of the defensive linemen are over both of the offensive guards and the center, thereby making it difficult to double team any one of the three interior defensive linemen. This can also take away the ability of the offense to pull the guards on a running play, because this would leave one of the defenders unblocked, or, at best, give another lineman a very difficult block to make on one of the defenders. The safety, like the linebackers, can blitz, play man-on-man, play zone, or drop back into deep coverage like a normal safety would do. The 46 is used in heavy run situations; when a team wants to bring lots of pressure; or merely to confuse the quarterback and offensive line.
In the 46 the outside linebacker on the strong side (the side with the tight end) lines up to play end and the two other linebackers hook the curl by dropping back at a 45 degree angle about 10-15 yards. Buddy Ryan is generally regarded as the originator of this defense, developing it and using it with great success with the 1985 Chicago Bears. It was named in honor of Chicago Bears safety, Doug Plank who wore the number 46.

4-4 defense

This rarely used defense is effective at run-stopping but is slightly weaker than a 4-3 defense at pass protection because it uses only three defensive backs. It is used more in college and high-school football than in the NFL.

Types of linebackers

There are several different designations of linebackers: strongside, middle, and weakside. Usually the strongside and weakside are combined under the title outside, and the middle is renamed inside. In many formations and systems teams do not use the strong and weakside designations, and merely play their outside linebackers consistently on one side of the formation and designate them either right outside linebacker and left outside linebacker. These terms are abbreviated ROLB and LOLB when appearing in lineup cards.

Outside linebacker

Strongside linebacker

The strongside linebacker (SLB) is often nicknamed "Sam" for purposes of calling a blitz. Since the strong side of the offensive team is the side on which the tight end lines up, or whichever side contains the most personnel, the strongside linebacker usually lines up across from the tight end. Often the strongside linebacker will be called upon to tackle the running back on a play, because the back will be following the tight end's block. He is most often the strongest linebacker, at the least he possess the ability to withstand, shed, and fight off blocks from a TE or a FB blocking the backside of a pass play. The linebacker should also have strong safety abilities in pass situation to cover the tightend in man on man situations. He should also have considerable quickness to read and get into coverage in zone situations.

Weakside linebacker

The weakside linebacker (WLB), or "Will", must be the fastest of the three, because he is often the one called into pass coverage. He is also usually chasing the play from the backside, so the ability to maneuver through traffic is a necessity for Will. Will usually aligns off the line of scrimmage at the same depth as Mike. Because of his position on the weakside, Will does not often have to face large interior linemen one on one unless one is pulling. In coverage, Will often covers the back that attacks his side of the field first in man coverage, while covering the weak flat or hook/curl areas in zone coverage. In a 3-4 defense the "Will" Linebacker plays on the "weakside" of the two middle Linebacker positions and a 4th Linebacker comes in to play the weakside. Known as a "Rush", "Rover", and/or "Buck" Linebacker, their responsibility is more pass rush based but often is called into run stop (gap control) and pass coverage.

Middle/Inside linebacker

Typically the middle linebacker (MLB), or "Mike", is responsible for calling the defensive play and communicating with the coach. The middle linebacker's primary responsibility is to be the lead tackler and shut down the opposition's running attack. In some defenses, Mike is responsible for a specific gap, while in others he's given more freedom. Because of his position directly over the ball, Mike must be able to effectively shed blocks coming from interior linemen but must also possess the range and speed to cover backs and shut down wide runs. Intense aggressiveness is often a desirable characteristic in a middle linebacker as hesitation of any duration can be fatal at this position. In the 3-4 defense, there are 2 different inside linebackers that occupy the middle, therefore the one closer to the strong side is called the "Mike", while the weak side is called "Will" and less often "Buck" or "Jack". Also Middle and Inside linebackers are known as LILB or RILB, which stands for Left Inside Linebacker and Right Inside Linebacker. This just depends on whether or not a linebacker is put in for the Middle Linebacker position when they usually play Outside Linebacker.

Receiving

Oddly, under goal line situations, some linebackers like Mike Vrabel will be lined up as an eligible receiver, on the spot normally taken by a tight end and catching short but critical scoring passes.